1.The Book
What is meant by the book is the Qurān which was sent by ammad (s).)Allah to the Prophet Muh
The Qurān that is in our hands today, its meaning and words has not been added to or taken away from.

“This Qur'an is not such as can be produced by other than Allah; on the contrary it is a confirmation of (revelations) that came before it, and a fuller explanation of the Book wherein there is no doubt - from the Lord of the worlds.”[29]
It is a holy book and all of the Muslims agree that it was divinely inspired and that everything inside of it is correct. This is the primary source of Islam and it takes away all the excuses that man can make all the way up to the Day of Judgment. It says that Allah's religion is Islam and that the Muslims must always follow the Qurān. It is a general law book for all of mankind.

The Authority of the BookIt is unanimous amongst the Muslims that the Qurān is an authority for Muslims. The proof behind this has two introductory statements:
1. Certainty that it was sent to the Prophet (s). This is established by multiple-successive reports passed down by Muslims from generation to generation.
2. The Qurān being sent by Allah. This is proved because of its miraculous nature in the way it is written and in its material. It is also proved by the fact that the Qurān dares mankind to bring something like it, but mankind is unable. Allah says:

ÊóäÒöíáõ ÇáúßöÊóÇÈö áóÇ ÑóíúÈó Ýöíåö ãöä ÑóøÈöø ÇáúÚóÇáóãöíäó

”(This is) the Revelation of the Book in which there is no doubt, from the Lord of the Worlds.”[30]

Jurisprudential Verses in Quran
There are around 500 verses in the Qurān that deal with religious rulings. These verses are a part of the sources for obtaining religious verdicts and are called: kām)ayyāt al-ah.

2. Traditions (Sunnah)The Arabic term sunnah literally means a way of acting, but figuratively it means: the words, actions and acceptances of an infallible. In order to understand this definition completely we must understand a few terms:
 Infallible: anyone who's infallibility is established. The people who are meant are the Prophet (s) and the 12 Imāms from the Ahl al-Bayt (a) if their infallibility is proved.
 The words of an infallible: Whatever the infallible says that has anything to do with religious rules.
 The actions of an infallible: Whatever actions an infallible performs.
 The acceptances of an infallible: Whatever actions are performed in front of an infallible when the infallible does not say anything about it.

The Authority of TraditionsAll of the Muslims are unanimous in the fact that the words, actions and acceptances of the Prophet (s) are considered an authority for all Muslims. Allah says:

æóãóÇ ÂÊóÇßõãõ ÇáÑóøÓõæáõ ÝóÎõÐõæåõ æóãóÇ äóåóÇßõãú Úóäúåõ ÝóÇäÊóåõæÇ

“So take what the Messenger assigns to you, and deny yourselves that which he withholds from you.”[31]
The words, actions and acceptances of the Imāms of the Ahl al-Bayt (a) are only considered as an authority after their infallibility and their successorship to the Prophet is proved. There are enough proofs of their infallibility from important sources found in books about Imāmat and theology. Refer to them.

3. Consensus
The Arabic term for consensus is ijmā‛ which literally means a strong-will.
The consensus that the Shia mention is no other than a tool to discover what the infallible's verdict is. Consensus is not an independent proof like the other three; the Qurān, traditions and intellect. Other jurisprudential sects believe that it is an independent proof.
Whenever a consensus shows us what the infallible's verdict is, it has authority. But, when it does not, it does not have authority.Question: Why do the Shia include this as one of the sources of religious verdicts when it is not independent?ārī answered this question in the following way:)Shaykh Ans including consensus in the sources for religious verdicts is a not being very precise. Consensus, with all of its conditions, discovers a proof and here both the discoverer and the discovered are called proofs.
How does a consensus discover the verdict of an infallible?This question is answered differently according to different opinions, which there are many of. Answers to this question started ūsī ® and continue to today. All of the answers have a)from the time of Shaykh T name and we will mention a couple of them here:
1. Internal consensus: A consensus of mujtaheds in an age informs us that an infallible was with them. He was part of the consensus but nobody knew him personally. So, this consensus is an authority. How do we know that the infallible was amongst them? This is answered in the books of the principles of jurisprudence.
2. Linguistic consensus: This consensus informs us, in an intellectual way, that the infallible agreed with the ruling but was not part of the consensus. His (a) duty is to prevent all of the scholars from making an incorrect consensus. More answers are found in the books of the principles of jurisprudence.

4. Intellect
What is meant by the intellect here is anything that man's intellect can understand and a religious ruling can be derived from.[32]
An example is when Allah makes obligatory an action through a Quranic verse or reliable tradition, but one must perform another action to be able to perform this obligatory action and there is not any verse or tradition about this action. Man's intellect understands the relationship between an obligatory action and its precepts becoming obligatory. This leads to certainty about the action being obligatory.
An example of this is that Allah made the pilgrimage obligatory on anyone who has financial ability. This is found in both the Qurān and traditions. But, Allah did not mention that the travel from one's hometown to Mecca is obligatory, even though it is a necessary precept to performing the pilgrimage.
Man's intellect understands the relationship between performing the pilgrimage and having to travel. It is possible to say that the travel becomes obligatory by the mukalaf having certainty, like some have said.

The Authority of Intellect
It is self-evident that intellect is an authority; it does not need a proof. The reason for this is that the intellect is a foundational proof for Islamic beliefs.
When it is considered as a foundational proof for Islamic beliefs it is easy to understating that it is an authority for religious rulings as well. The reason for this is that beliefs are more important than rules; they are the roots of religion.Notes:
[29] Yūnus: 37
[30] Sajdah: 2
[31] Hashr: 7
[32] alaqah 2, al-Dalīl al-‛Aqlī)adr, H)ammad Bāqir al-S)Muh